AC & High Voltage Transmission

This note covers the basics of alternating current (AC), high voltage transmission, transformers, electromagnetic induction, and the National Grid system used in the UK.

Alternating Current (AC)

Alternating Current (AC) is an electric current that continually changes direction. Unlike Direct Current (DC), where electrons flow steadily in one direction, AC reverses its direction many times per second.

In the UK, the mains electricity supply is AC with a frequency of 50 Hz. This means the current changes direction 50 times every second.

The voltage in AC also changes continuously, rising to a positive peak, falling to zero, then to a negative peak, and back again in a smooth wave pattern called a sine wave.

AC is preferred for electricity transmission because it is easier to change the voltage using transformers, which helps reduce energy loss when electricity is sent over long distances.

For instance, the mains supply in a home is AC at 230 V and 50 Hz, which powers most household appliances efficiently.

High Voltage Transmission

Electricity is transmitted across the country through power lines at very high voltages, often hundreds of thousands of volts. This is called high voltage transmission.

The main reason for using high voltage is to reduce energy loss as heat in the cables. When electricity flows, some energy is lost due to the resistance of the wires. This loss is given by the formula:

$$P = I^2 R$$

where $I$ is current and $R$ is resistance.

By increasing the voltage, the current needed to transmit the same power decreases (since power $P = VI$), which reduces the energy lost as heat.

The National Grid uses this principle to transmit electricity efficiently from power stations to towns and cities.

For example, if power of 1000 W is transmitted at 1000 V, the current is $I = \frac{P}{V} = \frac{1000}{1000} = 1 \text{ A}$. If the voltage was only 100 V, the current would be 10 A, causing much greater heat loss.

Transformers

Transformers are devices that change the voltage of an alternating current supply. They work only with AC because they rely on changing magnetic fields.

A transformer consists of two coils of wire, called the primary coil and the secondary coil, wound around an iron core.

When AC flows through the primary coil, it creates a changing magnetic field in the iron core. This changing magnetic field induces a voltage in the secondary coil by electromagnetic induction.

There are two main types of transformers:

• Step-up transformer: Increases voltage from primary to secondary coil (more turns on secondary coil).
• Step-down transformer: Decreases voltage from primary to secondary coil (fewer turns on secondary coil).

The voltages and number of turns on the coils are related by the transformer equation:

$$\frac{V_p}{V_s} = \frac{N_p}{N_s}$$

Where:

• $V_p$ = voltage across primary coil
• $V_s$ = voltage across secondary coil
• $N_p$ = number of turns on primary coil
• $N_s$ = number of turns on secondary coil

For example, if a transformer has 100 turns on the primary coil and 200 turns on the secondary coil, and the primary voltage is 230 V, the secondary voltage is:

$$V_s = V_p \times \frac{N_s}{N_p} = 230 \times \frac{200}{100} = 460 \text{ V}$$

Electromagnetic Induction

Electromagnetic induction is the process of generating a voltage (potential difference) across a conductor when it experiences a changing magnetic field.

This is the principle behind the generator effect, where moving a magnet near a coil of wire induces a voltage in the coil.

In transformers, the changing current in the primary coil produces a changing magnetic field in the iron core, which induces a voltage in the secondary coil.

Similarly, in AC generators, rotating a coil in a magnetic field causes the magnetic flux through the coil to change, inducing an alternating voltage and current.

The size of the induced voltage depends on:

• The speed of change of the magnetic field
• The number of turns in the coil
• The strength of the magnetic field

For example, increasing the speed of rotation in a generator increases the frequency and size of the induced voltage.

The National Grid

The National Grid is the system of cables, transformers, and substations that distributes electricity from power stations to homes and businesses across the UK.

Key components of the National Grid include:

• Power stations: Generate electricity, usually at a voltage of around 25,000 V.
• Step-up transformers: Increase voltage to hundreds of thousands of volts for transmission.
• High voltage transmission lines: Carry electricity over long distances with minimal energy loss.
• Step-down transformers: Reduce voltage to safer levels for local distribution (e.g., 230 V for homes).
• Substations: Control and distribute electricity to different areas.

Transformers are essential in the National Grid because they allow voltage to be increased for efficient transmission and decreased for safe use.

Safety is a major consideration: high voltages are dangerous, so power lines are kept high above the ground and insulated where necessary. Protective devices such as circuit breakers and fuses are also used to prevent damage and ensure safety. The system is designed for reliability, with multiple routes and backup systems to prevent blackouts.

Example: A power station generates electricity at 25,000 V. A step-up transformer increases this to 400,000 V for transmission. If the transformer has 500 turns on the primary coil, how many turns are on the secondary coil?

Using the transformer equation:

$$\frac{V_p}{V_s} = \frac{N_p}{N_s}$$

Rearranged to find $N_s$:

$$N_s = N_p \times \frac{V_s}{V_p} = 500 \times \frac{400,000}{25,000} = 500 \times 16 = 8,000$$

So, the secondary coil has 8,000 turns.